This invention relates generally to metal structural members for use in building construction, and more particularly to metal roof trusses for construction of roof framing for supporting roofs.
A roof truss generally comprises two or more top chord members and a bottom chord member. The ends of the top chords are secured together, and the ends of the bottom chord are connected to the lower, free ends of the top chords for forming the exterior of the roof truss. One or more web members span between and interconnect the top and bottom chords. The web members are secured at their ends to the top chords and to the bottom chord.
In building construction, a plurality of trusses are set out across a building frame. When erected upon the building frame, the bottom chord spans the wall frames of the building and is fixed to the top plate of the wall frames. The sub-roof material is then fastened to the top chords, and ceiling material may be fastened to the bottom chord. The combined load of the roof trusses, and the roofing and ceiling material attached to the trusses, is transferred through the outer edges of the trusses to the top plate of the wall frames.
In the past, roof trusses have been constructed of wooden chords and web members. More recently, various types of building systems incorporate metal trusses.
Metal trusses include chord members and web members rolled from metal sheets and formed into substantially rectangular U-shaped or C-shaped channels. The open sides of the chord members are adapted to receive the ends of the other chord members and the web members. The ends of the chords and web members are then fastened together for securing the truss elements in position. The materials cost for metal trusses is competitive with other building materials. Using metal as the material of construction also has a number of other advantages, including relatively stable price, strength, flexibility, durability, light weight, reliability, minimum waste in use, and noncombustability.
A significant problem with the use of metal trusses is the high installed cost. One factor influencing the installed cost of metal trusses is the thermal performance of metal, which is well below that of lumber framing when using standard framing techniques. This is due to the thermal conductivity of metal and the potential for thermal bridging. For example, steel conducts heat more than 300 times faster than wood. The rapid heat flow through steel reduces the insulating value of cavity insulation between 53 and 72%. With respect to metal roof trusses, heat passing through the ceiling material, if present, migrates into the bottom chord. Usually the bottom chord is covered with insulation spread on the attic floor, but heat can still be transferred into the truss at the points where the web members are fastened to the bottom chord. Heat is then conducted by the web members into the attic area and to the top chord at the underside of the roof. The result is a wicking effect whereby heat is transferred out of the building. Special considerations are necessary to reduce the tendency of metal roof trusses to transfer heat in this manner.
As a solution, some builders using metal wall frame construction, but top the building frame with wood roof trusses in order to minimize thermal bridging. However, this defeats the purpose of opting for metal frame construction. Other common solutions to improve energy efficiency include increasing the amount of cavity insulation and applying insulation to the exterior of the metal frame elements to provide a “thermal break” to the heat conducting path. Other means for reducing heat loss include punchouts in the chord members, wide truss spacing, and using thicker gauge steel. All of these approaches add to the cost, installation time and the difficulty of using metal roof trusses.
For the foregoing reasons, there is a need to provide a metal roof truss for use in a metal frame building system that is more energy efficient. Ideally, the new metal roof truss should be inexpensive, light weight, and adapted to mass production.